Sludge, generated during the course of conventional wastewater treatment, is usually de-watered (i.e. concentrated) prior to disposal via incineration, land application, land filling, composting, etc. A basic dewatering scenario involves forming strong, shear-resistant sludge flocs through the addition of a conditioning agent (e.g. ferric sulphate) and/or a flocculating agent (e.g. polyelectrolyte) followed by mechanical solid/liquid separation across gravity belt thickeners, belt filter presses, or centrifuges. By dewatering sludge, the wastewater treatment plant (WWTP) enhances the amount of solids per volumetric unit of sludge (i.e. cake solids) that ultimately must be disposed of. The benefits of higher cake solids include: Reduced dewatered sludge volume (less sludge to be “managed” by the plant); Lower annual transportation costs (shipping the sludge to landfills or sites of land application); Less water to be evaporated before sludge can be incinerated (increasing the net energy value of the sludge when incineration is used for cogeneration purposes); A more concentrated feed to digesters; and Reduced volume of sludge to be landfilled or land applied.
The generic composition of sludge is generally about 90-99% water, the remaining portion being total solids, with actual cell mass (i.e. bacterial cells) representing approximately 10% of the total solids. The remaining 90% of the total solids consists of extracellular polymeric substance (EPS) which forms a hydrated matrix within which the bacterial cells are dispersed. Sludge dewaterability, regardless of the means used to generate the sludge, has been largely associated with the EPS fraction of the whole sludge. EPS is comprised of debris from cell lysis (e.g. nucleic acid, lipids/phospholipids, protein, etc.), actively secreted extracellular products (e.g. polysaccharides and proteins), products of extracellular, EPS-bound enzymatic activity (e.g. polysaccharides), adsorbed material from the wastewater (e.g. humic substances, multivalent cations). Due to this complex nature of EPS and the predominant presence of polysaccharides and protein, EPS is traditionally characterized by the ratio of carbohydrates to proteins (EPScarb:prot). While the EPScarb:prot can vary from primary sludge to primary sludge depending on numerous operational parameters of the WWTP, the EPS composition within secondary sludges is somewhat more digestion specific: Anaerobically digested sludge EPScarb:prot tends to be less than unity while aerobically digested sludge EPScarb:prot is greater than unity. In any case, these primary components are considered to be the key hydratable substances within sludge flocs that effectively bind water and resist dewatering.
Methods which disrupt the water-binding capacity and/or mechanical integrity of sludge flocs are believed to enhance the dewaterability of the whole sludge upon polymeric flocculation. Most of such methods have focused on the ability of novel chemistries (e.g. acid pre-treatment, multivalent cationic conditioners) and processes (high temperature pre-treatment, electric discharge, sonication) to disrupt EPS components and improve dewaterability. A number of papers exist describing the use of enzymes for selective hydrolysis within the EPS to reduce the sludge volume, with varying results. See DE10249081, U.S. 2003014125, WO9110723, and DE3713739.